1. Field of the Invention
The present invention relates to an ink jet print head comprising a layer made by an improved curable resin composition, a process for the manufacturing thereof, and an improved curable resin composition.
2. Description of Related Art
The ink jet print head of an ink jet printer generally comprises a substrate, a barrier layer and a nozzle plate. The substrate is generally made of silicon. Various layers are deposited on a face of the silicon substrate to make up the ejection resistors and the active electronic components. The barrier layer is generally made of a photopolymer. Using photolithographic techniques, the ejection chambers and the microidraulic conducts for the ink delivery are realized in the photopolymer barrier layer. The nozzle plate is generally made of a plastic material, such as, for example, polyimide, or a metallic material, such as, for example, palladium plated nickel, rhodium plated nickel, or gold plated nickel. The nozzle plate provided with ejection nozzles made in correspondence with the ejection resistors and the ejection chambers is attached to the barrier layer.
In recent years, the nozzle plate has been made integrally with the barrier layer. The layer forming the barrier layer and the nozzle plate is known in the art as structural layer. In such a case, the manufacturing process includes a step of forming a pattern of the ejection chambers and the microidraulic conducts with a soluble resin or a metal, a step of coating a photopolymer covering the soluble resin or metal pattern, a step of forming orifices in the photopolymer in correspondence of the ejection chambers over the ejection resistors, a step of curing the photopolymer, and a step of dissolving the soluble resin or metal.
There are several problems which arise with respect to the photopolymer employed to make either the barrier layer or the structural layer.
The first problem is that the ink chemically attacks the photopolymer material and causes either leakage between the channels and/or leakage to the outside of the printheads and also causes swelling of the barriers. Swelling results in a change in channel geometry and a degradation from optimized performance. The problem of chemical attack is especially important with inks having pH's in excess of 7 and highly penetrating cosolvents.
The second problem is the resolution required upon patterning with a radiation source. The manufacturing of barrier or structural layers requires a resolution of approximately 20 micron which is in between the typical specifications of materials for printed circuit board, which have a resolution of approximately 200 micron and those for integrated circuits, which have a resolution of approximately 1 micron. The materials commonly used in the manufacture of printed circuit boards can not be used because these materials do not provide the high order of resolution that is required. If these materials are used to fabricate barrier or structural layers, the resulting layers are rough and granular. These are defects which cause unwanted flow discontinuities, obstructions and turbulence within the ink channels. On the other hand, the materials commonly used for integrated circuits are unusable because they are optimized to resolve dimensions on the order of 1 micron. When used to fabricate layers having thickness of approximately 25 microns, most integrated circuit materials lose all resolution. The material to be removed from the channels becomes too polymerized and can not be removed by conventional techniques.
The third problem is adhesion of the photopolymer layer to the substrate and/or to the nozzle plate. As mentioned above, conventional ink-jet printheads may comprise nozzle plates having a metal surface of palladium or gold. Also, the active electronic components realized on the substrate often comprise metal surfaces of gold or other materials showing low adhesion characteristics. Further, the adhesion of the photopolymer layer to the substrate and/or to the nozzle plate is also jeopardized by the mechanical strength of the photopolymer material, in particular when the manufacturing process of the print head requires thermal treatments. The thermal treatments promote the formation of mechanical stress which cannot be compensated by a material having high mechanical strength.
U.S. Pat. No. 5,150,132 describes an ink resistant material useful to make any component, particularly a top plate, of a printhead having a surface contacting the ink. The material is disclosed to have high glass transition point and excellent heat resistant properties. The component of the printhead is disclosed to be realized by molding, preferably by casting molding, compression molding or compression molding.
U.S. Pat. No. 5,478,606 discloses an ink-jet printhead comprising a structural layer formed by curing a radiation curable resin composition comprising an epoxy resin, such as, for example, those obtained from the reaction product between bisphenol A and epichlorohydrin, the reaction product between bromine-containing bisphenol A and epichlorohydrin, the reaction product between phenolic novolak or o-cresol novolak and epichlorohydrin, and the polyfunctional epoxy resins having oxycyclohexane skeleton. The resulting structural layer showed an excellent mechanical strength, adhesion and ink resistance. Moreover, in case of solid epoxy resin, at room temperatures, the patterning characteristics are also excellent.
U.S. Pat. Nos. 6,455,112 and 6,638,439 disclose the use of polyfunctional epoxy resins having oxycyclohexane skeleton to form structural layers of an ink-jet printhead.
U.S. Pat. No. 6,793,326 discloses that structural layers made of cationic polymerized product of alicyclic epoxy resins showed a peeling in case of high internal stress because of a high mechanical strength. The proposed solution suggests making the structural layer by curing a radiation curable resin composition comprising an epoxy resin having at least two epoxy groups and obtained from the polymerization of acrylic monomers bearing epoxy groups.
U.S. Pat. No. 6,193,359 discloses an ink-jet printhead comprising a barrier layer formed by curing a radiation curable resin composition comprising from 5 to 50 weight percent of a first multifunctional epoxy compound, typically a difunctional epoxy compound, from about 0.05 to about 20 weight percent of a second multifunctional epoxy compound, from about 1.0 to about 10 weight percent of a photoinitiator, and from about 20 to about 90 weight percent of a non photoreactive solvent. The cured composition is told to have greater resolution, higher aspect ratios, enhanced adhesion to metal surfaces, and resistance to ink chemical attack. However, the examples of the specification demonstrate that such a composition does not show a good adhesion to conventional nozzle plates.
The foregoing status of the art thus indicates that there is a need for a continuous research and improvement for a photopolymer layer material which can withstand the corrosion of high pH inks, provide the required ink channel resolution, stop delamination of the photopolymer layer from metal surfaces of gold or other metals showing low adhesion characteristics, and be easily employed in manufacturing processes.
The Applicant has noticed that when materials having a high glass transition point, particularly higher than 180° C., are employed to make a photopolymer layer, the high glass transition point gave to the material an excessive mechanical strength favoring the detachment of the layer from metal surfaces of gold or other metals showing low adhesion characteristics. Further, the use of molding techniques to realize the barrier or structural layer of a printhead is difficult and expensive in view of the reduced dimensions, in the order of some micrometers, connected with the manufacturing thereof.
Moreover, the Applicant has also noticed that the high mechanical strength and Tg of the epoxy resin radiation curable resin compositions promotes the detachment of the photopolymer layer from metal surfaces of gold or other metals showing low adhesion characteristics.
Additionally, the Applicant has also noticed that the epoxy resin obtained from the polymerization of acrylic monomers bearing epoxy groups did not show the optimal adhesion characteristics required to form a photopolymer layer of an ink-jet printhead.
Finally, the Applicant has noticed that the generally higher amount of difunctional epoxy compounds and solvent employed reduced the viscosity of the radiation curable resin composition so making it difficultly coatable by spin coating.